Brake adjusters

ABSTRACT

An encapsulated slack adjuster mechanism for a vehicle brake comprises an encapsulating container in which are disposed a nut having a non-reversible internal thread, an annulus encircling the nut, a reversible screwthread connection and cooperating friction surfaces. The reversible screw thread connection is between the annulus and the container or between the annulus and the nut whilst the friction surfaces are between the annulus and the nut or between the annulus and the container, respectively. A first spring acts on the annulus to urge the friction surfaces towards one another. A second spring acts on the second member and urges the latter into engagement by an abutment surface with a complementary non-rotatable abutment surface which may be on the container when the brake is not applied to avoid the nut turning when this is not desired. The first spring is in the container but the second spring may be within the container or may be external thereto. In the latter case the second spring can be a brake return spring.

United States Patent [1 1 Farr BRAKE ADJUSTERS Glyn Phillip ReginaldFarr, Kenilworth, England Inventor:

Girling Limited, Tyseley, Birmingham, England Filed: Aug. 7, 1972 Appl.No.: 278,355

[73] Assignee:

US. Cl. 188/196 D, 188/7l.9, 188/79 .5 P Int. Cl. Fl6d 65/56 Field ofSearch 188/71.9,'79.5 P,

188/79.5 GT, 196 D Primary ExaminerDuane A. Reger Attorney-SamuelScrivener et al.

[ Nov. 27, 1973 [57 ABSTRACT An encapsulated slack adjuster mechanismfor a vehicle brake comprises an encapsulating container in which aredisposed a nut having a non-reversible internal thread, an annulusencircling the nut, a reversible screwthread connection and cooperatingfriction surfaces. The reversible screw thread connection is between theannulus and the container or between the annulus and the nut whilst thefriction surfaces are between the annulus and the nut or between theannulus and the container, respectively. A first spring acts on theannulus to urge the friction surfaces towards one another. A secondspring acts on the second member and urges the latter into engagement byan abutment surface with a complementary non-rotatable abutment surfacewhich may be on the container when the brake is not applied to avoid thenut turning when this is not desired. The first spring is in thecontainer but the second spring may be within the container or may beexternal thereto. In the latter case the second spring can be a brakereturn spring.

23 Claims, 6 Drawing Figures BRAKE ADJUSTERS The present inventionrelates to brake slack adjusters of the kind described in British Pat.No. 1,179,235 and the corresponding US. Pat. No. 3,442,357 wherein aninterengagement through a helically inclined surface is used to convertexcessive axial displacement between two members into a rotary motionfor screwing a screw-thread connection to effect automatic adjustment.The invention relates more particularly to a slack adjuster of this kindin which the interengagement through a helically inclined surfacecomprises a reversible screw-thread connection.

In United States Patent application Ser. No. 64,518 there is described aslack adjuster for a brake system comprising a first non-rotatablemember, a second rotatable member and a third non-rotatable memberhavinga common thrust axis, a non-reversible screwthread connection betweensaid first and second members, said second member being axiallydisplaceable and rotatable relative to said third member and said secondmember having an abutment surface mating with a complementarynon-rotatable abutment surface; a fourth member coaxial with said thrustaxis; a reversible screw-thread connection between said fourth memberand one of said second and third members; cooperating friction surfacesbetween said fourth member and the other of said second and thirdmembers; and resilient means biassing said fourth member in a directionto urge said friction surfaces towards one another.

A reversible screw-thread connection between two members is a connectionsuch that axial displacement of one member causes rotation of the othermember and vice versa whereas a non-reversible screw-thread connectionbetween two members is one in which an axial force applied to one membercannot normally cause relative rotation between the members. Whether ascrew-thread connection is reversible or not depends upon the pitch andflank angles of the thread and the coefficient of friction between themembers.

Thus upon the presence of excessive brake slack, said resilient meansbiasses said friction faces to prevent said fourth member from rotatingand the axial travel between said first and third members upon actuationof the brake system causes said reversible screw thread connection toturn said second member to screw said non-reversible screw-threadconnection in a direction to reduce the brake slack.

In the embodiments illustrated in the abovementioned patent application,the second member is in the form of a sleeve which surrounds at leastparts of thefirst and fourth members and the fourth member is in theform of a screw member axially aligned with the first member and havinga domed head against which the resilient means acts with low frictionaltorque.

In accordance with one aspect of the invention, a slack adjuster for abrake system comprises a first nonrotatable member, a second rotatableor out member and a third non-rotatable member having a common axisthrust axis the first and second members having mating external andinternal threads respectively forming non-reversible screw-threadconnection between said first and nut members, said nut member beingaxially displaceable and rotatable relative to said third member; afourth or annular member coaxial with said thrust axis and encirclingsaid nut member; a reversible screw-thread connection between saidannular member and one of said nut and third members; cooperatingfriction surfaces between said annular memberand the other of said nutand third members} and resilient means axially biassing said annularmember in a direction to urge said friction surfaces towards oneanother.

The nut member will generally have an abutment surface mating with acomplementary non-rotatable abutment surface and the slack adjuster mayfurther comprise second resilient means biassing said nut member in adirection to urge said abutment surfaces into engagement with oneanother when the brake system is released in order to increase thefrictional torque resisting rotation of said nut member.

Advantageously the third member comprises an encapsulating containerwithin which are disposed the second member, the fourth member and theresilient means acting on the fourth member.

According to another aspect of the present invention, a slack adjustermechanism for a vehicle brake system comprises a nut member having aninternal screw thread adapted to mate with an external screw thread on astrut member to form a non-reversible screw thread connection, anencapsulating container member surrounding said nut member, said nut andcontainer members having cooperating axial abutment surfaces, an annularmember disposed within said container member and encircling said nutmember, said annular member and one of said nut and container membershaving a reversible screw thread connection therebetween, said annularmember and the other of said nut and container members havingcooperating friction surfaces therebetween, and resilient means disposedwithin said container member and axially biasing said annular member ina direction to urge said friction surfaces towards one another.

In accordance with some embodiments of the invention, wherein thereversible screw thread connection is between the annular member and thethird or container member, the reversible screw thread connectioncomprises an external thread on the outer periphery of the annularmember mating with an internal thread inside the third or containermember and the friction surfaces comprise an internal annular surfaceinside the annular member cooperating with an external friction surfacearound the nut member.

In other embodiments of the invention, wherein the reversible screwthread connection is disposed between the annular member and the nutmember, the reversible screw thread connection comprises an internalthread inside the annular member mating with an external thread aroundthe nut member and the cooperating friction surfaces comprise anexternal annular surface around the annular member cooperating with aninternal annular surface inside the third or container member.

The resilient means preferably comprises a spring washer bearing againstthe annular member.

The adjuster mechanism may be provided with an additional spring actingaxially on the nut member to bias the abutment surfaces against oneanother.

Said additional spring may be disposed in said container member to actbetween the nut and container members or it may be external thereto,e.g. as a brake return spring.

The invention is further described, by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a sectional elevation of an opposed piston hydraulic actuatorfitted with a slack adjuster in accordance with one embodiment of theinvention;

FIG. 2 is a sectional view of the slack adjuster mechanism of FIG. 1;

FIG. 3 is an end view of the slack adjuster mechanism of FIG. 2.

FIG. 4 is a sectional view, similar to FIG. 2, but showing a secondembodiment of slack adjuster mechanism in accordance with the invention;

FIG. 5 is an end view of the slack adjuster mechanism of FIG. 4; and

FIG. 6 is a sectional detail view of part of a drum brake which isfitted with a slack adjuster in accordance with a further embodiment ofthe invention.

FIG. 1 of the drawings shows opposed pistons and 21 of a hydraulicactuator for a disc brake such as a disc brake of the kind described andillustrated in British Pat. Nos. 1,075,371 and 1,147,639. The pistons 20and 21 are slidable in a through bore in a fixed body member. Aplate-like yoke straddles the disc and is slidably supported inlongitudinal grooves extending along opposite sides of the fixed bodymember. The piston 20 acts upon a directly operated pad assembly and thepiston 21 acts upon the yoke which in turn acts upon an indirectlyoperated pad assembly opposed to the directly operated pad assembly. Thebrake is applied hydraulically by supplying the brake fluid to the spacebetween the opposed pistons 20 and 21.

An auxiliary mechanical actuator (hand brake) comprises a cam 22journalled in the piston 21 and acting through a dolly 23 on a strut 24.The strut 24 acts in turn through an encapsulated slack adjustermechanism 25 in the piston 20. The strut 24 constitutes a first memberof the slack adjuster and has an external screw thread 26 mating with aninternal screw thread 27 in a nut 28 forming a second member of theslack adjuster.

The slack adjuster includes a third member in the form of a cylindricalencapsulating container 29 which is press fitted in a blind bore 30 inthe piston 20. The piston 20 can therefore be considered as also being apart of the third member of the slack adjuster.

The slack adjuster mechanism 25 is shown by itself in FIGS. 2 and 3 ofthe drawings. In addition to the nut 28 and the container 29 it alsoincludes a fourth member in the form of an annulus 31. The annulus 31 isdisposed within the container 29 and surrounds the nut 28. The annulus31 has multi-start external screw thread 32 mating with a multi-startinternal screw thread 33 inside the casing 29 to form a reversible screwthread connection 34. The reversible screw thread connection 34 is ofthe opposite hand to the non-reversible screw thread connection 35formed by the threads 26 and 27 i.e. assuming that the screw threadconnection 35 is a right hand thread then the reversible screw threadconnection 34 will be a left hand thread.

The annulus 31 has an internal conical friction surface 36 whichcooperates with an external conical friction surface 37 formed on aflange 38 at one end of the nut 28. To simplify assemblyduringmanufacture, the annulus 31 is provided with a second internal conicalsurface 39 directed oppositely to the surface 36 so that the annulus 31can be inserted in the container 29 either way round. Resilient means inthe form of a bowed washer spring 40 acts on the annulus 31 in adirection to axially bias the conical surfaces 36, 37 into frictionalengagement with one another.

An abutment surface 41 formed on one side of the flange 38 on the nut 28cooperates with a complementary abutment surface 42 formed on the insideof a ring 43 fitted inside the cylindrical container 29. The ring 43 isretained in position by a peened over edge 44 on the container 29 andthereby effectively forms part of the third member of the adjuster. Anadditional spring 45 acts on the nut 28 to axially bias the nut in adirection to bring the abutment surfaces 41 and 42 into engagement withone another.

The spring 45 acts between an internal flange 46 inside the container 29at the end remote from the ring 43 and a spring abutment cup 47. Theabutment cup 47 engages a frictional abutment surface formed by the sideof the flange 38 remote from the abutment surface 41.It serves to keepthe spring 45 away from the internal screw thread 33 inside thecontainer 29 and also defines an external recess to receive and locatethe bowed washer sring 40. Thus the spring 40' acts between the annulus31 and the spring abutment 47 but the spring 45 is stronger than thespring 40 so that the spring abutment remains in engagement with theflange 38. The strut 24 is sealed to the piston 21 by a sealing ring 48and is prevented from turning by a plate 49 which is attached to thepiston 21 by a spring ring and which engages in a flat or groove in thestrut.

It will be seen from FIGS. 2 and 3 that the encapsulated adjustermechanism 25 forms a complete replaceable sub-assembly but does notconstitute the whole of the slack adjuster inasmuch as the strut 24 isrequired to complete the adjuster. Also the same adjuster mechanism 25is suitable for pistons of different external diameters, it only beingnecessary to provide the pistons with a blind bore 30 of appropriatediameter to receive the adjuster mechanism 25 with the desired pressfit.

The normal brake travel between the pistons 20 and 21 is accommodated byaxial clearances at the screwthread connections 34 and 35. The axialclearance at the non-reversible screw thread connection 35 is thatnormally present due to manufacturing tolerances. Thus the reversiblescrew-thread connection 34 is provided with substantial axial clearanceto accommodate a preponderant proportion of the normal brake travel.

It will now be supposed that some pad wear has taken place so that thenext time the brake is applied hydraulically the travel between thepistons 20 and 21 exceeds a desired travel or slack as determined by thethread clearances.

The hydraulic pressure acting between the pistons 20 and 21 also acts onthe strut 24 to urge the strut to the right together with the piston 21.As the pistons 20 and 21 move apart the strut 24 pulls the nut28 to theright against the force of the spring 45. When the thread clearanceshave been taken up, the nut 28, pressing through the conical frictionsurfaces 36 and 37 also presses the annulus 31 to the right relative tothe container 29. Because of this relative axial movement, thereversible screw thread connection 34 causes the annulus 31 to rotate.Because of the spring 40 acting on the annulus 31 and because of theaxial force applied to the annulus 31 by the nut 28 at the conicalfriction surfaces 36 and 37 the frictional torque developed at thefriction surfaces 36 and 37 is greater than the frictional torquedeveloped at the side of the flange 38 on the nut 28 by the force of thespring 45. The annulus 31 thereby turns the nut 28 to effect adjustmentat the non-reversible screw thread connection 35 in a direction toreduce the brake slack. When the hydraulic pressure is released, thespring 45 acting as a strut return spring urges the nut 28 to the leftuntil the abutment surfaces 41 and 42 again come into engagement withone another. The leftward movement of the nut 28 relative to thecontainer 25 tends to part the conical friction surfaces 36 and 37 sothat the frictional torque developed at the conical friction surfaces isnow less than the frictional torque developed on the nut 28 by thespring 45. The spring 40, which is lighter than the spring 45, urges theannulus SI to the left so that the reversible screw thread connectioncauses the annulus 31 to turn in the opposite direction but the spring45 now prevents the nut 28 from turning so that slipping takes place atthe friction surfaces 36 and 37.

The dimensions of the various parts may be such that, when the mechanism25 is fitted in the piston 20, the bore end surface 88 in the pistonengages the end surface 89 of the nut 28 to keep the surfaces 41 and 42slightly parted. In this case, since the piston 20 is effectively partof the third member of the slack adjuster, the surfaces 89 and 88 takeover the role of abutment surfaces between the second and third members.

FIGS. 4 and 5 of the drawings show a different embodiment ofencapsulated brake slack adjusting mechanism 50.

In FIGS. 4 and 5 the multi-start reversible screw thread connection 34ais between the annulus 31a and the nut 28a and comprises an internalscrew thread inside the annulus 31a and an external screw thread on thenut 28a. The annulus 31a and the nut 28a are made of steel. An externalconical friction surface 360 on the annulus 31a cooperates with aninternal annular friction surface 37a defined by a step inside theencapsulating container 29a, which in this embodiment is made of pressedsteel. The spring 45 acts between an external flange 38a on the nut 28aand a washer 47a which bears against a frictional abutment surfaceformed by a second step inside the encapsulating container 29a. Thespring 40a-acting on the annulus 31a is disposed between this annulusand the washer 47a. The spring 45 acts in a direction to urge theabutment surfaces 41 and 42 on the flange 38a and inside the peened-overedge of the container 29a respectively into engagement with one anotherand the spring 40a urges the frictional surfaces 36a and 37a intofrictional engagement with one another.

The adjuster mechanism 50 of FIGS. 4 and 5 replaces the mechanism inFIG. 1. The encapsulating container 29a is push fitted inside the blindbore in the piston 20 and the strut 24 acting as the strut or firstmember of the adjuster is screwed into the internal screw thread 27 inthe nut 28a.

The operation of the adjuster mechanism of FIGS. 4 and 5 issubstantially equivalent to that of FIGS. 1 to 3. When the encapsulatingcontainer 29a moves to the left relative to the strut 24 (FIG. 1) duringbrake application and when there is excessive brake slack, the clearanceat the scew thread connections and 34a is taken up and the annulus 31ais moved to the left relative to the nut 280. Because this movementproduces an axial force applied to the annulus 310 through the frictionsurfaces 36a, 37a the frictional torque developed at the frictionsurfaces is greater than the frictional torque developed on the nut 28aby the spring 45, the annulus 31a is prevented from turning so that thereversible screw thread connection 34a causes the nut 28a to be turnedslightly to compensate for the excess travel. When the brake is releasedthe spring 45 returns the nut 28a until the abutment surfaces 41a and42a again engage one another but this time the frictional torquedeveloped at the friction surfaces 36a, 37a is less than that developedon the nut 28a by the spring 45 so that the annulus 31a turns back butthe nut 28a does not.

As in the first embodiment the normal brake travel is accommodatedpreponderantly by axial clearance at the reversible screw-threadconnection 34a, the axial clearance at the non-reversible screw-threadconnection 35 having no more than that dictated by usual manufacturingtolerances. v 7

Also as in the first embodiment the dimensions may be such that the endsurface 89 on the nut 28a engages the bore end surface 88 (FIG. I) toslightly part the surfaces 4la and 42a whereby the surfaces 89 and 88serve as abutment surfaces.

It will be noticed that in both embodiments the actual adjustment takesplace during brake application. By effecting various modifications itcan be arranged that the actual adjustment takes place upon brakerelease. This is achieved in each embodiment by making thenon-reversible and reversible screw thread connections of the same handand by making the conical friction surfaces 36, 37 or 36a, 37a slope theother way and transposing the spring 40 or 40a to the opposite side ofthe annulus 31 or 31a.

The adjusting mechanisms 25 and 50 are not damagedif the travel betweenthe pistons 20 and 21 should be greater than the travel permittedbetween the members of the mechanism. The encapsulating container issimply pulled by the strut 24 partially out of the bore 30. This mighthappen if the actuator is operated hydraulically with the brake padsremoved or if, for some reason new brake pads are replaced by worn brakepads. The encapsulating container is pressed home again by operation ofthe hand brake.

The adjuster mechanisms 25 and 56 of FIGS. 2 to 5 may be used inso-called swinging caliper disc brakes instead of the opposed pistondisc brake of FIG. 1. In this case the piston, in which the adjustermechanism 25 or 50 is fitted, is slidable in a bore in a body memberwhich comprises two parts of which one part contains said bore and theother part closes the end of this bore and contains a cam equivalent tothe cam 22 of FIG. 1. As in FIG. ll this cam acts through a dolly on astrut which has the non-reversible screw thread connection 35 with thenut 28 or 28a of the mechanism 25 or 50. The strut 24a is sealed to saidother part by a sealing ring like the ring 48 of FIG. 1 and is preventedfrom turning by a plate like the plate 49 which is attached to saidother part and which engages in flats or grooves in the strut. Thecaliper includes the two-part body member and can swing about an axisperpendicular to the disc axis. The axis of swinging may be parallel toor perpendicular to the line of the brake drag forces. The piston actson the directly operated pad assembly and the caliper which is rigidwith respect to the two part body member acts on the indirectly operatedpad assembly.

FIG. 6 illustrates an adjuster 66 for a drum brake having shoes (notshown) cooperating with a drum (also not shown). A hydraulic actuatoracts on the upper ends of the shoes whose lower ends rest againstabutments. An auxiliary mechanical actuator comprises a lever which actsbetween the upper end of one shoe and a strut assembly 65 acting on theupper end of the other shoe. A brake return spring (not shown) alsodraws the two upper ends of the shoes and towards one another.

The strut assembly 65 contains the automatic slack adjuster 66. Thestrut assembly comprises a first part 67 having an external screw thread26 mating with an internal screw thread 27 in a nut 28b to provide anonreversible screw thread connection 35. An encapsulating container 68for the adjusting mechanism is a press fit on one end of the second part69 of the strut assembly. The nut 28b has an external screw thread 32bwhich meshes with an internal screw thread 33b in an annulus 31b to forma multi-start reversible screw thread connection 34b of the same hand asthe nonreversible screw thread connection 35. The annulus 31b has anexternal conical friction surface 36b which cooperates with an internalconical friction surface 37b inside the container 68.

The not illustrated brake return spring urges an abutmentwasher 70 atthe right hand end of the nut 28b into abutment with the nut 28b. Thisserves to prevent the nut 281) from turning under the effects ofvibration. Resilient means in the form of a bowed washer spring 40b actsto the right on the annuus 31b to urge the friction surfaces 36b, 37binto engagement with one another and is disposed between the left handface of the annulus 31b and a ring 72 closing the cavity in thecontainer 66 and retained by a peened in lip 73. A rubber boot 75prevents dirt getting into the adjuster. A spring 74 acts between aninternal shoulder 71 inside the container 68 and the abutment washer 70and serves to cause the parts 67 and 69 of the strut assembly 65 tofollow the respective movements of the brake shoes during brakeapplication by the hydraulic actuator.

In the embodiment of FIG. 6 the normal brake slack is determined solelyby the axial clearance at the reversible screw thread connection 34b.The effect of the normal manufacturing clearance at the non-reversiblescrew-thread connection 35 is eliminated by the bias supplied to the nut28b by the brake return spring when the brake is not operated or by theadditional spring 74 when the brake is operated. During hydraulicactuation, the strut part 67 will move to the left relative to the part69 under the influence of the spring 74. When there is excess brakeslack, the axial clearance at the reversible screw-thread connection 34bis taken up. The frictional torque developed at the friction surfaces36b, 37b, and at the washer spring 40b is less than that resistingrotation of the nut 285 at the non-reversible screw thread connection 35and as a result the reversible screw thread connection 34b causes theannulus 31b to be turned slightly on the nut 28b. When the brake isreleased the two parts 67 and 69 again move towards one another but thistime the frictional torque developed at the conical frictional surfaces36b and 37b is increased to a value higher than that resisting rotationof the nut 28b. The reversible screw thread connection 33b now causesthe nut 28b to be turned on the strut part 67 to effect the automaticadjustment.

As in preceding embodiments it can be arranged that the adjustment takesplace during brake actuation rather than during brake release by makingthe screw thread connections 35 and 34b of opposite hand and byarranging the conical friction surfaces 36b and 37b to slope in theother direction, the spring 40b being transposed to the other side ofthe annulus 31b.

It will be seen that each of the illustrated embodiments of slackadjusting mechanism is compact and contains only a few parts, whichparts are comparatively small, especially those provided with thereversible screw thead. Also in each embodiment the adjusting mechanismforms a capsule ready to receive an externally threaded strut member.

The adjuster is thereby readily replaceable in the case of malfunction.

It will be noted that because the adjuster mechanism is a self-containedcapsule, a given capsule can be used for struts 24, 65, 67 of differentlengths according to the size of actuator to which the adjuster is to befitted.

Because the screw threads, and in particular the reversible screw threadconnection, are of a comparatively large diameter these can be producedby diecasting or by thread rolling. Also, because of this largerdiameter, the area transmitting the axial loading is greater so that acomparatively weak material can be used for the annulus 31, 31a, 31bsuch as zinc or brass rather than steel.

I claim:

1. A slack adjuster for a brake system comprising in combination a firstnon-rotatable member, a second rotatable nut member and a thirdnon-rotatable member having a common thrust axis, said first and nutmembers having axially external and internal threads, respectively,forming a non-reversible screw-thread connection between said first andnut members, said nut member being axially displaceable and rotatablerelative to said third member, a fourth annular member coaxial with saidthrust axis and encircling said nut member; a reversible screw-threadconnection between said annular member and one of said nut and thirdmembers; cooperating friction surfaces between said annular member andthe other of said nut and third members; and resilient means axiallybiassing said annular member in a direction to urge said frictionsurfaces towards one another.

2. A slack adjuster according to claim 1 in which said friction surfacesare in engagement with one another under the bias of said resilientmeans when the brake system is released.

3. A slack adjuster according to claim 1, said nut member having anabutment surface, further comprising a non-rotatable abutment surfacecomplementary thereto and second resilient means biassing said nutmember in a direction to urge said abutment surfaces into engagementwith one another when the brake system is released in order to increasethe frictional torque resisting rotation of said nut member.

4. A slack adjuster according to claim 3 in which said complementaryabutment surface is defined on said third member.

5. A slack adjuster according to claim 4 which further comprisesfrictional abutment surfaces additional to the previously mentionedabutment and friction surfaces, and in which said second resilient meanscomprises a spring which acts directly on said nut member at saidadditional abutment surfaces.

6. A slack adjuster according to claim 5 in which said spring actsbetween said nut and third members.

7. A slack adjuster according to claim 5 which further comprises acup-shaped abutment element for protecting said reversible screw threadconnection from said spring, said spring acts on said nut member throughsaid abutment element.

8. A slack adjuster according to claim 3 in which said second resilientmeans comprises a brake return spring external to the adjuster.

9. A slack adjuster according to claim 1 in which said third membercomprises an encapsulating container within which are disposed said nutmember, said annular member and said resilient means.

10. A slack adjuster according to cliam 4 in which said third membercomprises an encapsulating container within which are disposed said nutmember, said annular member, said first-mentioned resilient means andsaid additional resilient means.

11. A slack adjuster mechanism for a vehicle brake system comprising anut member having an internal screw thread adapted to mate with anexternal screw thread on a strut member to form a non-reversible screwthread connection, an encapuslating container member surrounding saidnut member, said nut and container members having cooperating axialabutment surfaces, an annular member disposed within said containermember and encircling said nut member, said annular member and one ofsaid nut and container members having a reversible screw threadconnection therebetween, said annular member and the other of said nutand container members having cooperating friction surfaces therebetween,and resilient means disposed within said container member and axiallybiassing said annular member in adirection to urge said frictionsurfaces towards one another.

12. An adjuster mechanism according to claim 11 which further comprisessecond resilient means acting axially on said nut member to bias saidabutment surfaces against one another.

13. An adjuster mechanism according to claim 12 in which said secondresilient means comprises a spring disposed within said container memberand acting between said nut and container members.

14. An adjuster mechanism according to claim 13 which further comprisesa cup-shaped abutment element for protecting said reversible screwthread connection from the spring, said spring acting on said nut memberthrough said abutment element.

15. A slack adjuster according to claim 1 in which at least one of saidfriction surfaces is conical.

16. A slack adjuster according to claim 1 in which said annular memberhas symmetrically oppositely tapering conical surfaces either one ofwhich can serve as said friction surface on said annular member, wherebysaid annular member may be inserted either of opposite ways round insaid third member.

17. A slack adjuster according to claim 1 wherein said reversible screwthread connection is between said annular member and said third memberand comprises an external thread on the outer periphery of said annularmember and an internal thread inside said third member and wherein saidfriction surfaces comprise an internal annular surface inside saidannular member and an external friction surface around said nut member.

18. A slack adjuster according to claim 1 wherein said reversible screwthrew connection is disposed between said annular member and said nutmember and comprises an internal thread inside said annular member andan external thread around said nut member and wherein said cooperatingfriction surfaces comprise an external annular surface around saidannular member and an internal annular surface inside said third member.

19. A slack adjuster according to claim 1 in which said resilient meanscomprises a spring washer bearing against said annular member.

20. A slack adjuster according to claim 3, wherein said reversible screwthread connection is between said annular member and said third memberand comprises an external thread on the outer periphery of said annularmember and an internal thread inside said third member and wherein saidfriction surfaces comprise an internal annular surface inside saidannular member and an external friction surface around said nut memberand which further comprises an abutment element by'which said secondresilient means acts on said nut member, said first-mentioned resilientmeans comprising a spring washer acting between said annular member andsaid abutment element.

21. A slack adjuster according to claim 3 wherein said reversible screwthread connection is disposed between said annular member and said nutmember and comprises an internal thread inside said annular member andan external thread around said nut member and wherein said cooperatingfriction surfaces comprise an external annular surface around saidannular member and an internal member surface inside said third memberand which further comprises a washer held in place in said third memberby said second resilient means, said first-mentioned resilient meanscomprising a spring washer acting between said annular member and saidwasher.

22. A slack adjuster according to claim 1 in which said reversible screwthread connection comprises mating multistart threads.

- 23. A slack adjuster according to claim 1 in which said reversiblescrew-thread connection has substantial axial clearance, said axialclearance accommodating at least a preponderance of the desired normalbrake slack.

1. A slack adjuster for a brake system comprising in combination a firstnon-rotatable member, a second rotatable nut member and a thirdnon-rotatable member having a common thrust axis, said first and nutmembers having axially external and internal threads, respectively,forming a non-reversible screw-thread connection between said first andnut members, said nut member being axially displaceable and rotatablerelative to said third member, a fourth annular member coaxial with saidthrust axis and encircling said nut member; a reversible screw-threadconnection between said annular member and one of said nut and thirdmembers; cooperating friction surfaCes between said annular member andthe other of said nut and third members; and resilient means axiallybiassing said annular member in a direction to urge said frictionsurfaces towards one another.
 2. A slack adjuster according to claim 1in which said friction surfaces are in engagement with one another underthe bias of said resilient means when the brake system is released.
 3. Aslack adjuster according to claim 1, said nut member having an abutmentsurface, further comprising a non-rotatable abutment surfacecomplementary thereto and second resilient means biassing said nutmember in a direction to urge said abutment surfaces into engagementwith one another when the brake system is released in order to increasethe frictional torque resisting rotation of said nut member.
 4. A slackadjuster according to claim 3 in which said complementary abutmentsurface is defined on said third member.
 5. A slack adjuster accordingto claim 4 which further comprises frictional abutment surfacesadditional to the previously mentioned abutment and friction surfaces,and in which said second resilient means comprises a spring which actsdirectly on said nut member at said additional abutment surfaces.
 6. Aslack adjuster according to claim 5 in which said spring acts betweensaid nut and third members.
 7. A slack adjuster according to claim 5which further comprises a cup-shaped abutment element for protectingsaid reversible screw thread connection from said spring, said springacts on said nut member through said abutment element.
 8. A slackadjuster according to claim 3 in which said second resilient meanscomprises a brake return spring external to the adjuster.
 9. A slackadjuster according to claim 1 in which said third member comprises anencapsulating container within which are disposed said nut member, saidannular member and said resilient means.
 10. A slack adjuster accordingto cliam 4 in which said third member comprises an encapsulatingcontainer within which are disposed said nut member, said annularmember, said first-mentioned resilient means and said additionalresilient means.
 11. A slack adjuster mechanism for a vehicle brakesystem comprising a nut member having an internal screw thread adaptedto mate with an external screw thread on a strut member to form anon-reversible screw thread connection, an encapuslating containermember surrounding said nut member, said nut and container membershaving cooperating axial abutment surfaces, an annular member disposedwithin said container member and encircling said nut member, saidannular member and one of said nut and container members having areversible screw thread connection therebetween, said annular member andthe other of said nut and container members having cooperating frictionsurfaces therebetween, and resilient means disposed within saidcontainer member and axially biassing said annular member in a directionto urge said friction surfaces towards one another.
 12. An adjustermechanism according to claim 11 which further comprises second resilientmeans acting axially on said nut member to bias said abutment surfacesagainst one another.
 13. An adjuster mechanism according to claim 12 inwhich said second resilient means comprises a spring disposed withinsaid container member and acting between said nut and container members.14. An adjuster mechanism according to claim 13 which further comprisesa cup-shaped abutment element for protecting said reversible screwthread connection from the spring, said spring acting on said nut memberthrough said abutment element.
 15. A slack adjuster according to claim 1in which at least one of said friction surfaces is conical.
 16. A slackadjuster according to claim 1 in which said annular member hassymmetrically oppositely tapering conical surfaces either one of whichcan serve as said friction surface on said annular member, whereby saidannular member may be inserted either of opposite ways round in saidthird member.
 17. A slack adjuster according to claim 1 wherein saidreversible screw thread connection is between said annular member andsaid third member and comprises an external thread on the outerperiphery of said annular member and an internal thread inside saidthird member and wherein said friction surfaces comprise an internalannular surface inside said annular member and an external frictionsurface around said nut member.
 18. A slack adjuster according to claim1 wherein said reversible screw threw connection is disposed betweensaid annular member and said nut member and comprises an internal threadinside said annular member and an external thread around said nut memberand wherein said cooperating friction surfaces comprise an externalannular surface around said annular member and an internal annularsurface inside said third member.
 19. A slack adjuster according toclaim 1 in which said resilient means comprises a spring washer bearingagainst said annular member.
 20. A slack adjuster according to claim 3,wherein said reversible screw thread connection is between said annularmember and said third member and comprises an external thread on theouter periphery of said annular member and an internal thread insidesaid third member and wherein said friction surfaces comprise aninternal annular surface inside said annular member and an externalfriction surface around said nut member and which further comprises anabutment element by which said second resilient means acts on said nutmember, said first-mentioned resilient means comprising a spring washeracting between said annular member and said abutment element.
 21. Aslack adjuster according to claim 3 wherein said reversible screw threadconnection is disposed between said annular member and said nut memberand comprises an internal thread inside said annular member and anexternal thread around said nut member and wherein said cooperatingfriction surfaces comprise an external annular surface around saidannular member and an internal member surface inside said third memberand which further comprises a washer held in place in said third memberby said second resilient means, said first-mentioned resilient meanscomprising a spring washer acting between said annular member and saidwasher.
 22. A slack adjuster according to claim 1 in which saidreversible screw thread connection comprises mating multistart threads.23. A slack adjuster according to claim 1 in which said reversiblescrew-thread connection has substantial axial clearance, said axialclearance accommodating at least a preponderance of the desired normalbrake slack.